Actually, you can say しつれいします both on entering and leaving. On entering, 1 knock 2 wait the boss to say "Please come in. " 3 say しつれいします 4 open the door and enter. 5 close the door quietly, bow When leaving, 1 wrap up whatever the conversation you had with your boss with something like「ありがとうございました」or「よろしくおねがいします」 2 make sure the boss is expecting you to leave 3 say「では、しつれいします」, go to the door, bow, open the door, leave, close the door quietly. しつれいしました　is used when you did something inappropriate mistakenly.

When you enter the room. しつれいします。 and when you leave the room. しつれいしました.。←　past form (?)

there is many way today end of greeting it is different what happen on the that boss office Just bring in someting "失礼しました”　something instruction "ありがとうございました”　something you make mistake " すいませんでした　each case same thing is when you out room you have to say past tence. good luck